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Thursday, August 16, 2007

How the Meter Came To Be

Author: James Monahan

One can know where one is in the world by the systems of measurement that specific place uses. There is the English system used by the United States, which uses pounds and feet for measurement, and then there is the metric system which is more accepted in other parts of the so-called civilized world.

While there are three types of systems of units of use today, the most popular one by far is the International System of Units (or the SI Systeme International d'Unites).

A measurement in this particular system with regards to length is in meter/metre. Variations in the meter are prefixes such as kilometer and millimeter. The word has Greek roots, its origin being metron, which means ""a measure"".

The meter follows a timeline dating back to the eighteenth century, when two approaches to the definition of the standard unit of length were broached.

The first approach defined the meter as the length of a pendulum with a half-period of one second. The other approach suggested that the meter was one-fourth the polar circumference of the earth.

On May 8, 1790, the French National Assembly approved of the first approach: its length would be equal to the length of a pendulum with a half-period of one second.

Barely a year later, in March 30, 1791, has this same assembly accepted the new proposal of the French Academy of Sciences which adhered to the second approach: that the new definition of the meter would be equal to one-fourth the polar circumference of the world.

It must be noted that the circumference of the Earth, if measured through the poles, is about forty million meters.

In December 10, 1799, the French National Assembly then specified that the final standards would be according to the platinum meter bar constructed on June 23rd 1799 and currently deposited in the National Archives.

In the 1870's a series of international conferences were held to devise new metric standards. It was the Meter Convention of 1875 that mandated the establishment of an enduring International Bureau of Weights and Measures (or BIPM, for Bureau International des Poids et Mesures) to be based in France.

It was this organization that was tasked to uphold the new prototype kilogram and meter when it would be constructed. It would also retain comparisons between the distributed metric prototypes and the non-metric measurement standards.

Almost a decade later, in September 28, 1889 the CGPM defined the length as the exact distance between two lines on a standard bar of an alloy of platinum with ten percent iridium. This distance was to be measured at the melting point of ice.

This definition would be adjusted over the years. It was in 1893 when Albert A. Michelson, the inventor of the interferometer, measured the standard meter using his device. It won't be until 1925 when interferometry would be in regular use at the BIPM.

On October 21st, 1983 the seventeenth CGPM definition of a meter equaled the length traveled by light in vacuum during 1/299, 2972, 458 of a second.

Scientists agree that if a definition is based on the physical properties of light, then it is infinitely more precise and reproducible. This is because the properties associated with light are considered to be universally constant.

About the author: James Monahan is the owner and Senior Editor of MeterIndex.com and writes expert articles about meters .